CLIC damping rings overview and open issues

1. CLIC damping rings overview and open issues Yannis PAPAPHILIPPOU October 16th-18th, 2007

2. Outline M. Korostelev (PhD thesis, EPFL 2006) • CLIC damping rings design and open issues • Design goals and challenges • Input parameters - Pre-damping rings • Lattice choice and optics optimisation • Circumference (realistic drift space and magnets) • Wiggler design and parameter scan (prototypes) • Synchrotron radiation absorption • Chromaticity correction and dynamic aperture • Low emittance tuning in the presence of coupling (tolerances) • e-cloud and other collective effects • Summary CLIC Workshop '07

3. R ~ 130 m e- Main Linac e+ Main Linac e- BC2 e+ BC2 12 GHz 2.4 GV 12 GHz 2.4 GV 12 GHz, 100 MV/m, 21 km 12 GHz, 100 MV/m, 21 km RTML RTML L ~ 1100 m 9 GeV 48 km 3 TeV Base line configuration (L. Rinolfi) Booster Linac 6.6 GeV 3 GHz  500 m  100 m  100 m e+ BC1 e- BC1  5 m  5 m 30 m 30 m 3 GHz 88 MV 3 GHz 88 MV 2.424 GeV 365 m 2.424 GeV 365 m e+ DR e- DR e- PDR e+ PDR 2.424 GeV 2.424 GeV Injector Linac 2.2 GeV 1.5 GHz  220 m  230 m  30 m e-/e+ Target Pre-injector Linac for e+ 200 MeV Laser Thermionic gun Unpolarized e- Pre-injector Linac for e- 200 MeV Positron Drive beam Linac 2 GeV DC gun Polarized e- 1.5 GHz 1.5 GHz 1.5 GHz  5 m  15 m  200 m Injector complex

4. Damping ring design goals • Ultra-low emittance and high beam polarisation impossible to be produced by conventional particle source: • Ring to damp the beam size to desired values through synchrotron radiation • Intra-beam scattering due to high bunch current blows-up the beam • Equilibrium “IBS dominated” emittance should be reached fast to match collider high repetition rate • Other collective effects (e.g. e--cloud) may increase beam losses • Starting parameter dictated by design criteria of the collider (e.g. luminosity), injected beam characteristics or compatibility with the downstream system parameters (e.g. bunch compressors)

5. CLIC Pre-damping rings • Pre-damping rings needed in order to achieve injected beam size tolerances at the entrance of the damping rings • Most critical the positron damping ring • Injected emittances ~ 3 orders of magnitude larger than for electrons • CLIC PDR parameters very close to those of NLC (I. Raichel and A. Wolski, EPAC04) • Similar design may be adapted to CLIC • Lower vertical emittance • Higher energy spread L. Rinolfi

6. CLIC damping ring layout CLIC Workshop '07

7. TME arc cell • TME cell chosen for compactness and efficient emittance minimisation over Multiple Bend Structures (or achromats) used in light sources • Large phase advance necessary to achieve optimum equilibrium emittance • Very low dispersion • Strong sextupoles needed to correct chromaticity • Impact in dynamic aperture

8. Phase advance choice • Optimum horizontal phase advance of cells for minimising zero current emittance is fixed (284o for TME cells) • Vertical phase advance is almost a free parameter • First iteration based on lattice considerations, i.e. comfortable beta functions and relaxed quadrupole strengths and chromaticity • Low horizontal phase advance gives increased momentum compaction factor (high dispersion) but also chromaticity CLIC Workshop '07

9. Phase advance with IBS Horizontal phase advance for minimum horizontal emittance with IBS, is found in an area of small horizontal beta and moderate dispersion functions (between 1.2-1.3π, for CLIC damping rings) Optimal vertical phase advance quite low (0.2π) The lowest longitudinal emittance is achieved for high horizontal and low vertical phase advances The optimal point has to be compromised due to chromaticity considerations and dynamic aperture optimisation CLIC Workshop '07 9

10. Circumference Drifts + dipoles • Big enough to accommodate bunch train • Drift space increase essential for establishing realistic lattice, reserving enough space for instrumentation and other equipment • For constant number of dipoles (TME cells), zero equilibrium emittance is independent of circumference • Normalised emittance with IBS increases with circumference (no wigglers) • When dipole lengths increase with drifts, emittance grows due to increase of damping time (inversely proportional to radiation integral I2 which decreases with length) • When only drifts increase, smaller emittance growth due to increase of optics functions • Impact on chromaticity + dynamic aperture • Compensation may be achieved due to increase of bunch length with circumference (momentum compaction) • Linear optics has to be reviewed with realistic magnet parameters Only Drifts Drifts + dipoles Only Drifts CLIC Workshop '07

11. Damping wigglers • Damping wigglers are used to increase radiation damping and reduce the effect of IBS in order to reach target emittances • The total length of wigglers is chosen by its dependence with the peak wiggler field and relative damping factor Fw • Higher damping factor can be achieved for higher fields and longer wiggler occupied straight section • Relative momentum spread is independent of total length but increases with wiggler field CLIC Workshop '07

12. Wigglers effect in emittance For fixed value of wiggler period, equilibrium emittance minimum for particular value of wiggler field By reducing total length, optimal values necessitate higher fields and lower wiggler periods Optimum values change when IBS included, necessitating higher fields Damping rings cannot reach 450nm with normal conducting wigglers CLIC Workshop '07

13. Wigglers’ effect with IBS For higher wiggler field and smaller period the transverse emittance computed with IBS gets smaller The longitudinal emittance has a different optimum but it can be controlled with the RF voltage BINP PM wiggler BINP SC wiggler ANKA SC wiggler • The choice of the wiggler parameters is finally dictated by their technological feasibility. • Normal conducting wiggler of 1.7T can be extrapolated by existing designs • Super-conducting options have to be designed, built and tested CLIC Workshop '07

14. Wiggler prototypes • Two wiggler prototypes • 2.5Τ, 5cm period, built by BINP • 2.7Τ,2.1cm period, built by ANKA • Wiggler prototype of 2.6T, 4cm period • Aperture of 8-10mm • Current density can be increased by using different conductor type • Establish field error tolerances for both designs • Short version to be installed and tested at ANKA (GADGET) • Measurements of emittance growth due to IBS in a wiggler dominated ring (ANKA at injection energy of 0.5 GeV)

15. Synchrotron Radiation and losses • Regularly distributed short absorbers with apertures of 4-6mm considered for SR power of around 11kW based on the old parameters (reduced bunch charge, increased number of bunch trains, permanent magnet wiggler) • Need of a terminal absorber at the end of the straight for absorbing 90 kW • For the new parameters, beam current of 0.17A produces synchrotron radiation of around 9.2kW in the high-field superconducting wiggler (7.9kW for the lower field) • Considering a scheme with a long bunch train filling the whole ring, the above values are raised by a factor of 7! • Review absorber design, considering super-conducting wiggler quench protection and fit it into ring layout

16. Non-linear dynamics • Two sextupole schemes • 2 and 9 families of sextupoles • Dynamic aperture is 9σxin the horizontal and 14σy in the vertical plane (comfortable for injection) • Error tables for all magnets including superconducting wigglers should be considered and optimised • Resonance correction and DA optimisation with sextupoles and/or octupoles using modern techniques (normal forms, frequency maps, ...) CLIC Workshop '07

17. Coupling correction • Correction with dispersion free steering (orbit and dispersion correction) • Skew quadrupole correctors for correcting dispersion in the arc and emittance minimisation • Iteration of dynamic aperture evaluation and optimisation after correction • In CLIC damping rings, the effect of vertical dispersion is dominant (0.1% of coupling and 0.25μm of dispersion invariant) • Effect of super-conducting wigglers field errors • Review of linear correction systems (orbit, beta variation, coupling) with realistic magnet parameters and re-establishment of alignment tolerances

18. e--cloud effect • Simulations with ECLOUD code revealed the importance of the effect in both CLIC and TESLA damping rings D. Schulte, R. Wanzerberg , F. Zimmerman, ECLOUD’04 • Simulations using the FAKTOR2 code confirmed the importance of the effect (W. Bruns and G. Rumolo, CLIC meeting 06/06/2007) • Ante-chambers in dipoles and wigglers need to absorb 99.9% of photon flux • Secondary emission yield has to be less than 1.3 • e-cloud density of 3-5x1012 m-3 in the wigglers (independently of density in dipoles) for beam to be stable • Simulations have to be carried out for the newest parameter set • Bunch population of 4.1x109 • Bunch spacing from 0.667 to 0.5ns • Wiggler field of 2.5T (or 2.7T) • Inclusion of linear optics in HEADTAIL simulations and effect of non-linear chromaticity ρwig = 5x1012m-3, ρdip = 3x1011 m-3 CLIC Workshop '07

19. e--cloud countermeasures • Coating of vacuum chambers by a material (e.g. NEG) for lowering secondary emission yield (GADGET) • Clearing electrodes • Solenoids in field-free regions • Grooved surface of vacuum chamber • Simulations showing reduction of SEY • Verified experimentally in PEPII • Slight resistive wall impedance increase L. Wang et al., PAC2007 courtesy by M. Pivi

20. Other collective effects Longitudinal and micro-wave instability Keil-Schnell-Boussard threshold is higher than scaled values from the KEKB LER Transverse coasting beam instability associated with transverse impedance Detailed vacuum chamber design and impedance budget Coherent Synchrotron radiation has a minor effect of only 5% of bunch lengthening not causing any emittance blow-up or microwave instability Space-charge tune-shift is higher than the acceptable value of 0.1 To be taken into account in non-linear dynamics and working point choice Fast ion instabilities Analytical estimates and simulations for old parameter set assuming total pressure of 1nTorr (20% CO, the rest hydrogen) Ionisation cross-section of 0.2nTorr and 30% ion frequency spread Ion accumulation avoided for train gap of a few meters Review with new parameter set Toushek lifetime large enough compared to store time Resistive wall growth time estimated to around 2ms Couple bunch instabilities have to be avoided with design of HOM free RF cavities Y.P., 18/10/2007 CLIC Workshop '07 20

21. Damping rings’ parameters • 2005: original ring • 2006a: super-conducting wiggler considered • 2006b: vertical dispersion included • 2007a: 12GHz structure • 2007b: reduced bunch population • 2007c: CLIC_G structure CLIC Workshop '07 CLIC PWG Y. Papaphilippou 21

22. Bunch charge • For the CLIC damping rings, the horizontal normalized emittance scales approximately as • The above relationship is even more exact when the longitudinal emittance is kept constant (around 5000 eV.m, in the case of the CLIC damping rings) • Vertical and longitudinal emittance weakly dependent on bunch charge, and linear with each other • Numerical tools have to be optimised for evaluation of final emittance with IBS • IBS theory and numerical tools have to be reviewed for non-Gaussian tails • Demonstration of low-emittance in the presence of IBS needs to be proved experimentally CLIC Workshop '07 22

23. Concluding remarks • Detailed and robust design of the CLIC damping rings, delivering target emittance with the help of super-conducting wigglers • Prototype to be built and tested at ANKA synchrotron • Radiation absorption and quench protection • Areas needing further optimisation and/or detailed studies • Pre-damping ring optics design • Realistic damping ring cell length and magnet parameters • Sextupole optimisation and non-linear dynamics including wiggler field errors • Linear and non-linear correction schemes • Low emittance tuning and alignment tolerances • IBS theory, numerical tools and experimental demonstration of low emittance • Collective effects including electron cloud and fast ion instability • Detailed vacuum chamber design – impedance budget • Injection and extraction elements • Design of HOM free high frequency RF cavities • Diagnostics and feedback CLIC Workshop '07